138 ATOMS, IONS, SALTS, AND SURFACES 



ory of Einstein, the conversion of hydrogen into helium should liberate an enormous 

 amount of energy. The reaction may be written: 



4H^He 



4.0312-gm. mass-^4.ooo-gm. mass+o.o3i2-gm. mass in the form of radiation 

 0.0312-gm. mass is equivalent to 0.0312X9X10^° ergs = 

 2.81X1C ergs = 6.71 X 10" calories 



In more ordinary terms, the conversion of i lb. of hydrogen into helium should give 

 off as much energy as radiation — which could be transformed into heat — as would be 

 given by the burning of 10,000 tons of coal. 



The frequency of light- vibrations is very great. According to the quantum theory, 



if this frequency is multiplied by a con- 

 stant //, the quantum constant, the 

 energy of one quantum of the radiation 

 Q is obtained, E = hv. Conversely, the 



energy quantity divided by h gives the 

 frequency of the vibration, which is the 

 Q velocity of light (c) divided by the 



wave-length (X). From these relations 

 x-v we find that if four hydrogen atoms 



^^ were to be converted into one helium, 



Q all of the particles having small veloci- 



ties, the wave-length emitted would be 

 0.00046 A, or this would be of the type 

 of the cosmic radiation which falls upon 

 the earth from outside. The reaction 

 Fig. 4 -Neutral sodium atom (diagrammatic probably occurs in several steps, but a 



O 



representation in a plane: the structure is un- ■ r ^u 1 ^\, v.^ 1 i. u 



^.^^^^^^,i part of the wave-lengths emitted should 



be of this general order of magnitude. 



known). 



IONS 



Atoms, or molecules which are not electrically neutral, are called "ions," so ions 

 contain either a larger or a smaller number of electrons than of protons. The non- 

 nuclear, or planetary, electrons of an atom are classified in sets. Thus in the sodium 

 atom there are supposed to be three of these sets : the innermost with two, the next 

 with eight, and the third or outermost with one electron. These numerical relations 

 are represented by Figure 4. The chemical properties of an element are supposed to 

 depend to a large extent, but not wholly, upon the number of electrons in the outer set. 

 Thus the atoms of the alkalies, lithium, sodium, potassium, rubidium, and caesium, 

 are all supposed to have one electron in the outer set. They ionize, i.e., lose this outer 

 electron, more easily than any other elements. All of them react violently with water, 

 and in all of their other chemical, and in most of their physical, properties act almost 

 alike. 



With helium the outer set of electrons is complete when two electrons are present, 

 but with heavier elements completeness in this set seems to be reached only when 

 eight electrons are present. 



